Numerous insects are serious pests of common agricultural crops. One method of controlling insects has been to apply insecticidal organic or semiorganic chemicals to crops. This method has numerous, art-recognized problems. A more recent method of control of insect pests has been the use of biological control organisms which are typically natural predators of the troublesome insects. These include other insects, fungi (milky-spore) and bacteria (Bacillus thuringiensis cv., commonly referred to as "Bt"). However, it is difficult to apply biological control organisms to large areas, and even more difficult to cause those living organisms to remain in the treated area for an extended period. Still more recently, techniques in recombinant DNA have provided the opportunity to insert into plant cells cloned genes which express insecticidal toxins derived from biological control organisms such as Bt. This technology has given rise to additional concerns about eventual insect resistance to well-known, naturally occurring insect toxins, particularly in the face of heavy selection pressure, which may occur in some areas. Thus, a continuing need exists to identify naturally occurring insecticidal toxins which can be formed by plant cells directly by translation of a structural gene.
The lectin from Bauhinia purpurea is a glycoprotein the cDNA-derived amino acid sequence of which indicates a polypeptide chain comprising 262 amino acids that include 7 lysine residues. See, e.g., "cDNA Cloning and Expression of Bauhinia purpurea Lectin," Kusui et al., J. Biochem. 109, 899-903 (1991). This lectin has been determined by Czapla et al. to have larvicidal activity against European corn borer, as disclosed in the copending, commonly assigned U.S. patent application "Larvicidal Lectins and Plant Insect Resistance Based Thereon," filed Sep. 20, 1991, Ser. No. 07/763,100. One of us is also a coinventor of that application. Other insects show little susceptibility to this lectin, and acquired resistance to naturally occurring lectins is likely to exist. Thus, a continuing need is felt for new larvicidal proteins which are not found in nature, yet can easily be expressed in plant cells as a gene product of a single structural gene.